1. Field
The present disclosure relates generally to wireless communications and more specifically to a method and apparatus for providing reliable multicast in a Wireless Local Area Network (WLAN).
2. Background
WLANs have become popular and are used in many homes and businesses. WLANs can be found in places like coffee shops, Internet cafes, libraries, and public/private organizations. With the increase use of WLANs, demand for broadcasting/multicasting has also steadily increased. Thus, a need exists for reliable multicasting.
Users want to be able to watch videos, receive newscasts, and other local broadcast information. A commonly used communication protocol in a WLAN is the IEEE 802.11, which has the ability to multicast by utilizing a broadcast or multicast address within the MAC layer. However, the 802.11 protocol does not support reliable multicasting, in that, the multicast frames are not acknowledged e.g. via acknowledgment (ACK) signaling and are not retransmitted.
Multicast frames are frames transmitted by an access point (AP) with the receiver address set to one of a set of broadcast or multicast addresses. A multicast address represents a group of recipients. The AP maintains only one multicast queue and associates a single sequence number space with all multicast and broadcast user data frames. When multicast frames are transmitted with no ACK policy, their frame loss rates can be quite large. A loss rate of 10% is fairly typical. Frame losses can occur not only due to the signal-to-noise ratio (SNR) distribution depending on the location of the stations (STA) within the AP coverage area (e.g., the distance of the STA from the AP), but also due to overlapping transmissions, hidden terminal problems, etc. Although the coverage can be improved by lowering the data rate chosen for the frame transmission, using a lower data rate results in the frame transmission occupying the medium for a longer duration, thus increasing the likelihood of getting interfered by an overlapping transmission. As a result, it is unlikely that the frame error rate of unacknowledged transmissions on an 802.11 WLAN can be reduced significantly. Thus, when the multicast frames are lost and the protocol does not retransmit, the service becomes unacceptable to users.
In addition, a problem occurs in legacy devices if original multicast or broadcast frames are retransmitted by the AP. A legacy device, for example, may be one that uses any 802.11 protocol not modified according to the invention described here. The problem is that in a legacy device the MAC layer will not detect the duplicates causing the upper layers to be confused by the duplicates. In other words, a legacy STA will pass the received multicasted frame up to the receiver stack to the application regardless if the legacy STA has received the same frame previously. This will cause the STA to see out of order and duplicate packets. This problem exists independent of what mechanism is introduced to cause an AP to transmit multiple copies multicast frames. Thus, there is a need to provide support for legacy STAs as well as for newer STAs that can benefit from retransmissions of the multicast or broadcast frames.
A proposed solution to these problems have been to allow full acknowledgements. Allowing full acknowledgements of the multicast frames may result in wasted bandwidth from acknowledgements transmitted by all devices subscribed to the multicast stream and increased complexity of defining a protocol to obtain acknowledgments from all devices.
In order to overcome these problems, the present disclosure proposes a solution wherein the AP can make multiple retransmissions of frames associated with a multicast stream without causing existing devices to see out-of-order and duplicate frames, while providing improved reliability for newer devices and working within the existing protocols.
Other benefits, features and advantages of the various aspects will become apparent from the following detailed description, figures and claims. It should be understood, however, that the detailed description and the specific examples, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.